Hands Free Telephone Systems (HFTS) can be used with little or no use of the hands, leaving the user's hands free for other tasks (such as driving). The HFTS is connected to a conventional mobile telephone which in turn is connected to the wireless telephone network. The HFTS may exchange audio signals with the mobile telephone over a wireless connection (e.g. Bluetooth), or alternatively, the telephone device may be snapped into a fixed cradle and get the audio signals over a solid connection (electrical contacts). In some applications, the mobile telephone is represented by a network access device (NAD) which might be integrated together with the hardware for the RIFTS within the same unit.
FIG. 1 shows an example of a typical vehicle application where a vehicle enclosure 100 contains the vehicle's speakers 104 that reproduce the audio of a telephone call and an embedded phone microphone 105 that is placed somewhere in the vehicle enclosure 100 to capture the caller's voice. HFTS 101 is connected to the vehicle speakers 104 and phone microphone 105, and also by a Bluetooth connection to a conventional mobile telephone 102 in the vehicle. The mobile telephone 102 is connected to the wireless telephone network 103 over a digital radio communication link where a radio-frequency (RF) signal is transmitted. The mobile telephone 102 encodes the audio signals for transmission to the telephone network 103 using a speech codec which accomplishes both source coding and channel coding. Various specific speech codecs are available depending on the type of wireless telephone network 103 and its load. Examples of speech codecs include without limitation the enhanced variable rate codec (EVRC) of the CDMA network that is used in North America and the adaptive multirate codec (AMR) and the enhanced full-rate codec (EFR) which are both used for the GSM network in Europe.
Since the HFTS 101 replaces the speaker and microphone of the mobile phone 102, many of the same audio issues arise that conventional mobile phones and land phones have to deal with, for example, echo cancellation and noise suppression. HFTS 101 also incorporates a signal processing unit that in part aims to enhance the signal quality of the system dealing with issues such as disturbances to the microphone signal by background noise, coupling of the loudspeaker output back to the phone microphone 105 (echo component), and other audio interferences. There also might be multiple phone microphones 105 (microphone arrays) and/or multiple vehicle speakers 104, in which case, the signal processing unit might also include multi-channel signal processing such as beam forming and/or multi-channel echo cancellation. Moreover, a phone call made from the vehicle enclosure 100 may have to deal with high levels of ambient noise entering into the call. Thus, any acoustic optimization solution must not only clearly transmit the caller's voice, but also remove these high levels of noise. An overview of the signal processing methods that can be applied within HFTS 101 can be found, for example, in Acoustic Echo and Noise Control, E. Hänsler and G. Schmidt, Wiley 2004, incorporated herein by reference.
To approach acoustic optimization of the HFTS 101, a measurement system is used that replaces the actual wireless telephone network 103 with a network simulator that communicates with the mobile telephone 102 over the same interface as the actual network does. The network simulator decodes the encoded speech signal received from the mobile phone 102 and it encodes the return signal sent back to the mobile phone 102 using the codec type and its parameters that are specified by the network. Evaluation of the HFTS 101 is typically based on the VDA standard (Verband der Automobilindustrie e. V. (VDA): VDA Specification for Car Hands-free Terminals, Edition 1.6, November 2008, incorporated herein by reference), which provides a widely recognized specification that aims to ensure a comfortable conversational quality to the user. The VDA standard specifies, for example, the signal levels in the send and receive directions, and the required echo suppression performance.
FIG. 2 shows a typical example of an HFTS measurement arrangement based on using the VDA standard. The HFTS 101 transmits and receives audio signals to and from the mobile phone 102. For a VDA measurement, the actual wireless telephone network is replaced by a network simulator 201 which communicates with the mobile phone 102 over the same interface as the actual network. Measurement system 202 receives the audio signal of the mobile phone 102 from the network simulator 201 and also may generate a return audio signal back through the network simulator 201 to the mobile phone 102. The measurement system 202 also is connected to a special measurement speaker 204 and a measurement microphone 203 which are temporarily placed inside the vehicle for the measurements. The measurement speaker 204 can simulate a local talker (e.g. by a head-and-torso simulator, HATS), and with the measurement microphone 203 the local audio output of the HFTS 101 can be evaluated.
FIG. 2 also shows further details of the HFTS 101 which typically includes an analog-to-digital converter (ADC) 207 that digitizes the incoming analog microphone signal for signal processing unit 210, and a digital-to-analog converter (DAC) 208 that generates an analog speaker output signal, which also may go to an external amplifier 209 before it is output by the vehicle speaker 104. Sample rate converters (SRC) 205 convert the digital data stream to and from the Bluetooth interface 206 to be compatible with the Bluetooth codec that reduces the amount of errors and/or the amount of data. Besides a Bluetooth link as shown in FIG. 2, the connection between the HFTS 101 and the mobile phone 102 can be realized in other specific ways such as Wireless LAN, infrared, hardwire interface where the mobile phone 102 is clamped in a cradle with a plug having electrical contacts. Or, the mobile phone 102 may be integrated into a network access device (NAD) together in the same unit with the hardware for the HFTS 101. The connection between the mobile phone 102 and the network simulator 201 uses a speech codec 211 which performs source coding to reduce the amount of information and channel coding to make the transmission robust to errors. The speech codec 211 loses signal information and thus reduces the quality of the speech signal.
The VDA standard specifies that the HFTS audio signals are analyzed after transmission over a simulated network as described above. Thus, standard state-of-the-art measurement systems require expensive hardware (in particular the network simulator). In addition, those measurement standards evaluate only the signals that are observable on the far-end side (audio signals to/from the network simulator) and on the near-end side (audio signals within the car enclosure). But these signals give only limited insight into the behavior of the hands-free telephone system, as the behavior of the mobile telephone also is included.